Remotely powered transponder

ABSTRACT

A transponder having a circuit for the extraction of power from an incident interrogating beam of electromagnetic energy, the extracted power being utilized to operate a digital coding circuit. The transponder further comprises an oscillator circuit for developing a train of pulses of electromagnetic energy with successive pulses occurring in a coded format in accordance with a digital code imparted by the digital coding circuit. The transponder is of sufficiently small size to be affixed in the form of a tag to automobiles, personnel, containers and other objects to be identified. The electronic tag communicates with an interrogation system.

S tes Pate 1 [111 3,745,569 Works et al. 1 1 July 10, 1973 [54] REMOTELYPOWERED TRANSPONDER 3,377,616 4/1968 Auer 343/65 SS 5] n e rs eo g A. oJo C u r y, 3,389,391 6/l968 Keeler et al 343/68 R both of Hudson;Edward D. Ostrofi, I Sudbury; Nathan Freedman, West PrimaryExaminer-Stephen C. Bentley Newton an of Mass Attorney-Milton D.Bartlett, David M. Warren et all. [73] Assignee: gkdaaygtsheon Company,Lexington, ABSTRACT [22] Filed. July 22 1971 A transponder having acircuit for the extraction of power from an incident interrogating beamof electro- PP -Z 165,219 magnetic energy, the extracted power beingutilized to operate a digital coding circuit. The transponder furj S'Cl343/65 SS ther comprises an oscillator circuit for developing a [51] in}CL. G015 9/56 train of pulses of electromagnetic gy with Succes [58] R 65 LL sive pulses occurring in a coded format in accordance 5 SS 8 8 witha digital code imparted by the digital coding circuit. The transponderis of sufficiently small size to be [56] References Cited affixed in theform of a tag to automobiles, personnel,

containers and other objects to be identified. The elec- 2 927 321PATENTS 343/6 8 R tronic tag communicates with an interrogation system.

arrls 3,391,404 7/1968 Vogelman 343/68 R 27 Claims, 12 Drawing FiguresPATENTEU Jul 1 0 SHEET 5 BF 7 TO COUNTER 2I4 l I l ZOIDF/G. 9

E L I I 1 I I FROM DIODE IZGA DIODE ARRAY 202 REMOTIELY POWEREDTRANSPONDEIR BACKGROUND OF THE INVENTION mally sensitive element in itstuned circuit for retransmitting a pulse of electromagnetic energy at afrequency slightly offset from that of the interrogating frequency suchthat the amount of frequency offset is related to the temperature assensed by the thermally sensitive element. Or, alternatively, suchtransponders may comprise a tuned circuit responsive to some harmonic orsubcarrier frequency of the interrogating signal for responding with apulse of electromagnetic energy at that frequency.

A problem exists in that in many applications today, such as in theidentification of automobiles in a car rental system, o'r in theidentification of packages moving along a conveyer belt, or in theidentification of medicine contained in bottles in a hospital, as wellas numerous other similar type situations requiring identity of anobject as well as information about the object, it is frequentlydesirable to provide a substantial amount of data in a manner which issuitable for rapid display and for entry into a computer. For example,in an automobile rental system it is desirable to know the identity ofthe car, the mileage, the amount of gasoline, the expected date ofreturn and similar matters; all of this to be extracted in therelatively short interval of time that is required for the automobile topass by a gate of the parking lot. Similar comments apply to the storageof medicine in a bottle in a hospital wherein it is desirable to knowthe identity of the medicine and the patient for whom it has beenprepared, the attending physician, the patients location in thehospital, the date and similar matters; all this data to be provided bya momentary scan of the bottle of medicine to insure effective operationof hospital administrative procedures.

SUMMARY OF THE INVENTION The aforementioned deficiencies in theinformational capacity of transponders energized from the energy of aninterrogating signal are overcome by a transponder and interrogator inaccordance with the present invention in which the transponder utilizesa succession of serially connected antenna elements, each of which iselectrically connected with a rectifying device, preferably a microwavediode, for developing a source of power having a voltage substantiallyhigher than that provided by a single radiating element. A low voltageoscillator circuit, such as a tunnel diode relaxation oscillator, isenergized by successive pulses of an interrogating electromagnetic beamincident upon one of the aforementioned antenna elements, an in responsethereto, provides a succession of pulses of electromagnetic energy via aseparate antenna. For example, the interrogating beam may be at anX-band microwave frequency while the response is provided at L-band. Thetransponder further incorporates a preset memory system, preferably anarray of conductors arranged in a matrix with preselected crosspointscoupled by diodes, the conductors being energized via counters inresponse to a counting of the interrogating pulses, and the output ofthe memory being utilized to activate (or inhibit) the oscillatorcircuit to provide a succession of the radiant energy pulses in adigitally coded format for transmission of the message stored in thememory. The transponder is of sufficiently small physical size to beattached to an object in the form of a tag.

The interrogator incorporates means for identifying a preset sequence ofpulses to establish that a transponder is present as well as to identifythe beginning of the message. In addition the interrogator furtherchecks each pulse with the pulse arriving one message interval earlierto further identify the presence of a valid message. The message isstored, as by a shift register having parallel outputs, to provide readyaccess to a display or computer.

BRIEF DESCRIPTION OF THE DRAWINGS The aforementioned features and otheradvantages of the invention are explained in the following descriptiontaken in connection with the accompanying drawings wherein:

FIG. I is a description of a cargo identification system in whichpackages tagged with transponders pass before an interrogator inaccordance with the invention for identification of the individualpackages;

FIG. 2 is a block diagram of the interrogator of the invention includinga communication link with the transponder of the invention;

FIG. 3 is a diagrammatic view, partially schematic and partiallystructural, of the transponder of the invention;

FIG. 4 is a detailed view of the back side of the transponder showinginterconnections between the antenna elements;

FIG. 5 is a schematic diagram of the interconnection of antenna elementsof the transponder;

FIG. 6 is an oscillator circuit of the transponder for transmitting aresponse to the interrogator;

FIG. 7 shows the current-voltage relationship for a tunnel diode of theoscillator circuit;

FIG. 8 is a graph of the tunnel diode voltage, and further indicates theinterval of oscillation;

FIG. 9 is a schematic diagram of a counter and decoder utilized in thecoding circuit of the transponder for coding the response with datastored within the transponder;

FIG. 10 is a dimetric view of an alternative antenna arrangement for thetransponder utilizing spiral antenna elements;

FIG. 11 is a schematic diagram for switchably altering the cross-pointconnections in the memory system of the transponder; and

FIG. 12 shows a means for automatically imprinting a message in thememory system of the transponder.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to FIG. 1, thereis shown a pictorial view ofa package identification system 20 in whichindividual packages 22 are transported along a conveyer belt 25 in ashipping room 26. The individual packages are identified, in accordancewith the invention, by means of transponders 28 each of which has theform of a tag which is affixed to a package 22. An interrogator 30interrogates each transponder 28 as the packages 22 pass by theinterrogator 30. A message, relating to the contents of an individualpackage 22 and stored within the transponder 28, is transmitted back tothe interrogator 30 and appears on a display 32 connected by electricalcable 33 to the interrogator 30 and conveniently mounted on a shelf 34for easy viewing by personnel of the shipping room 26. The interrogator30 is shown mounted on a tripod 36 so that it may be readily positionedto interrogate the individual packages 22 as they pass along theconveyer belt 24.

The interrogator 30 is equipped with a directional transmitting antenna38 and a directional receiving antenna 40 so that electromagnetic energymay be directed towards a single transponder 28 even when the packages22 are closely spaced. The interrogator 30 is equipped with a thresholdcircuit, to be described hereinafter, whereby the interrogator 30 isadjusted for the intensity of radiation transmitted back from atransponder 28 to the receiving antenna 40 for improved discriminationbetween the transmissions of transponders 28 on closely spaced packages22. The intensity of the power transmitted by the transmitting antenna38 is also adjustable, in a manner to be described, so that atransponder 28 is radiated with an optimum intensity independently ofthe distance between the interrogator 30 and the conveyer belt 24.

To provide isolation between the transmitting antenna 38 and thereceiving antenna 40, it is convenient to transmit and receive onseparate frequencies. Thus, in the preferred embodiment the transmittingantenna 38 operates at X-band (approximately 9.4 GHZ) and the receivingantenna 40 operates at L-band (approximately 1.2 GHz); circular orlinear polarization may be utilized. Alternative antenna configurationsfor the transponders 28 adapted for linear or circular polarizationswill be disclosed hereinafter. As shown in FIG. 1, the horn of thetransmitting antenna 38 is equipped with diagonal vanes to providecircular polarization, and the receiving antenna 40 includes a helicalradiating element for reception of circularly polarized radiation. Thebeam of radiation from the transmitting antenna 38 is indicated by rays46. While the linear polarization has the advantage of providing asimpler antenna configuration within the transponder 28, the circularpolarization is advantageous in that there is no requirement that thetransponder 28 be positioned in a preferred direction to intercept thepolarized radiation.

The radiant energy signal transmitted from a transponder 28 to theinterrogator 30 consists of a digitally coded pulse train, to bedescribed hereinafter, which conveys the message stored within thetransponder 28 to the interrogator 30. The digital code includes asequence of digits which identify the presence of a valid signalradiated by a transponder 28, as well as indicating the beginning andend of the digitally coded message. The remainig digits of the codedpulse-train signal are utilized to convey the message itself. Thus, aseach transponder 28 moves into the rays 46 and, in response thereto,begins retransmitting to the interrogator 30, the interrogator 30processes the received pulse-train signal, in a manner to be described,and stores the message for a few moments as it is displayed on thedisplay 32. The interrogator 30 then again processes the receivedpulse-train signal and, if the transponder 28 is still within the rays46, this information is displayed for a few more moments on a display32; however, if the transponder 28 on the next package 22 has moved intothe rays 46, then the message in that transponder is displayed on thedisplay 32.

While the transponder to be described herein is responsive to incidentelectromagnetic energy, it is understood that a similar type transpondermay be provided which is responsive to incident acoustic energy. Forexample, a beam of pulses of radiant acoustic energy incident upon apiezoelectric crystal is converted by the crystal to pulses ofelectrical energy which may then be processed in the same manner as willbe dis,- closed hereinafter with reference to the transponder responsiveto the incident electromagnetic energy.

Referring now to FIG. 2, there is shown a block diagram of theinterrogator including the X-band and L- band transmission links withthe transponder 28. The signal transmitted by the transmitting antenna38 to the transponder 28 is a pulse modulated X-band carrier signal inwhich the succession of pulses occur respectively in response to clockpulses on line 48 from a timing unit 50. In response to each of theclock pulses on line 48 a pulse generator 52 generates a pulse having awidth on the order of one microsecond which is then applied to atransmitter 54. The transmitter 54 is a well known X-band source ofelectromagnetic energy which is modulated by the pulses of pulsegenerator 52 to provide a succession of X-band pulses along line 56 tothe directional antenna 38. The transmitter 54 is equipped with a poweroutput control 58 for adjusting the power of the signal on line 56 in awell known manner such as by adjusting an attenuator (not shown) withinthe transmitter 54. The repetition rate of the clock pulses on line 48is high enough to accommodate the data rate in the transmission of themessage from the transponder 28 to the interrogator 30; for example, arepetition rate of 4 KHz is utilized in the preferred embodiment.

The transponder 28 transmits along the L-band link to the receivingantenna 40 a pulse train signal in which each pulse corresponds to oneof the transmitted pulses from the transmitting antenna 38. The codingof the transponder output signal, as well be described hereinafter, isaccomplished by deleting certain pulses in each sequence of pulses inaccordance with a digital format in which the presence of a pulsecorresponds to a logic 1 and the absence of a pulse corresponds to alogic 0. Since the transponder 28 has its own oscillator circuit forgenerating pulses of microwave energy, the pulses in the L-band linkhave a pulse width independent of the pulse width of pulses in theX-band link, and furthermore the occurrence of the pulses in the L-bandlink is delayed from that of the pulses in the X-band link. However, thepulse repetition frequency of pulses in the L-band link is equal to thatof pulses in the X- band link.

The pulses of microwave energy in the L-band link incident upon thereceiving antenna 40 are received by a receiver 60 which amplifies,filters and demodulates these pulses in a well known manner to provide apulse signal on line 62. The bandwidth of the receiver 60 is provided inaccordance with well known procedures for passing the major spectralcomponents of the pulses and attenuating noise in the spectral regionoutside the spectrum of the pulses.

The pulse signal on line 62 is compared to a reference signal 64 bymeans of a comparator 66 which provides an output sequence of pulses online 68 corresponding to the sequence of pulses on line 62 when theamplitude of the pulses on line 62 is greater than that of the referencesignal 64. The reference signal 64 is adjusted by control 69 (apotentiometer, not shown, may be utilized to provide the referencesignal) in accordance with the anticipated amplitude of signals receivedat the interrogator 30, of FIG. 1, from a transponder 28 directly infront of the receiving antenna 40. This value of the reference signal 64is larger than a signal which would be received from a secondtransponder 28 substantially off the axis of the receiving antenna 40.While it is doubtful that a transponder 28 located substantially off theaxis of the transmitting and receiving antennas 38 and 40 would receivesufficiently intense radiant energy to be activated to retransmit thecoded message, the use of the comparator 66 provides an additionalsafeguard such that only one transponder 28 is interrogated at a giventime.

It is desirable to render the interrogator 30 nonresponsive to receivedsignals during those instance of time when transmitted pulses appear online 56, thereby insuring that there is no interference betweentransmitted and received signals. This function is accomplished by meansof multivibrators 70, 72, and flipflop 74. The multivibrator 70 istriggered by the clock pulses on line 58 so that the flip flop 74receives a reset signal corresponding to each of the clock pulses 48.

As has been noted, the transponder 28 does not transmit immediately uponreception of the X-band signal, but transmits after a small delay which,as will be seen, is in the range of 5-30 microseconds depending on theintensity of the incident X-band radiation. Furthermore, in view of theshort range between the interrogator 30 and the transponder 28 as isevidenced by the arrangement of the shipping room 26 of FIG. 1, thepropagation delay of signals on the X-band and L-band links are normallywell under 100 nanoseconds so that the only delay that need beconsidered is that provided by the transponder 28 itself. Accordingly,the multivibrator 70 is monostable for providing a delay somewhat lessthan the delay in response time of the transponder 28, for example 3microseconds, this delay being accomplished by triggering themultivibrator 72 and the flip-flop 74 on the trailing edge of the pulsesignal on line 76 provided by the multivibrator 70. Thus, the flipflop74 is reset immediately before the arrival ofa pulse signal on line 68,and is then set by a pulse on line 68 if such pulse has not been deletedas part of the digital coding. Accordingly, there appears on line 78 avoltage provided by flip-flop 74 corresponding to a logic state of 11 ifa pulse has arrived on line 68, while the voltage on line 78 remains ata value corresponding to a logic state of O in the event that the pulseon line 68 has been deleted in accordance with the digital coding of themessage of the transponder 28. Thus, with respect to the digital wordappearing on line 68, logical states of l appearing on line 78 are incorrespondence with the logical states of l appearing on lines 68, andlogical states of 0 appearing on line 78 correspond to the logicalstates of 0 appearing on line 68.

Multivibrator 72, a monostable multivibrator which is similarlytriggered by the trailing edge of the pulse on line 76, provides inresponse to each of these triggerings a clock pulse signal on line 80for flip-flop 82 as well as an optional gating signal on line 84 for thereceiver 60. The clock pulse on line 80 is a negative going pulseprovided by the Q output of the multivibrator 72, while the gatingsignal on line 84 is a positive pulse provided by the Q output of themultivibrator 72. As has been mentioned above, it is desirable to renderthe interrogator 30 nonresponsive to signals which may be incident uponthe receiving antenna 40 except at such times as when a pulse may bereceived from the transponder 28. The multivibrator 72 provides a pulsewidth somewhat larger than the width of pulses appearing on line 62 and,accordingly, the signal on line 84 is well suited for gating thereceiver to render it responsive to incoming signals at such times as apulse is expected from transponder 28. Gated receivers are well knownand need not be further described. In the preferred embodiment of theinvention the pulse width of the multivibrator 72 is approximately 40microseconds since, as will appear presently, the pulse width of pulsesgenerated by the transponder 28 is typically 5 microseconds.

The flip-flop 82 is of the form known as a D-edge flipflop whichfunctions in a manner analogous to a one bit shift register such that apulse signal appearing at terminal D is momentarily stored and thenpresented at terminal Q on line 86 in response to the pulses on lineapplied to the clock input terminal of the flip-flop 82. Thus, when thesignal voltage on line 80 is of a high value, a signal at the terminal Dis stored in the flip-flop 82; and when the signal voltage on line 80 isof a low value, the voltage at terminal Q of flip-flop 82 assumes a highor a low value corresponding to the high or low value of the signalstored in the flip-flop 82. As was mentioned above, the voltage on line78 is either of a low value corresponding to the logic state of 0 inresponse to a reset condition of the flip-flop 74, or alternatively, thevoltage on line 78 is of a high value corresponding to a logic state of1 in response to a set condition of flip-flop 74. It is noted that a setcondition of flip-flop 74 is retained from the instant of reception of apulse signal on line 68 until after the generation of the next clockpulse on line 48. By way of summary it is seen, therefore, that a pulsesignal on line 68 appears as a voltage level on line 78 which persistsfor approximately the interpulse spacing of the clock pulses on line 48,and furthermore, the voltage level on line 78 reappears on line 86 aftera delay of approximately the interpulse period of the clock pulses online 48.

The pulse signal on line 84 is also applied via AND gate 88 to serve asclock pulses on line 90 for clocking shift register 92. In response tosuccessive clock pulses on line 90, the shift register 92 acceptssuccessive inputs of the voltages appearing on line 86, that is, thehigh voltage on line 86 being stored in shift register 92 as logic 1 andthe low voltage appearing on line 86 being stored in shift register 92as logic 0. In the preferred embodiment of the invention, the message ofthe transponder 28 has a word length of 40 bits of which the first 8bits form a sequence of 0s and ls which is unique in the message, thatis, this particular sequence is not found anywhere else in the messageword. The last 32 bits of the message word represent the actual datacontent of the message. Accordingly, each bit of the message word isseen to repeat once every 40 bits, or, equivalently, once for every 40pulses transmitted on the L-band link from the transponder 28.

In the processing of the message by the interrogator 30, two tests areperformed. A random digit test is performed to determine that all digitsin the shift register are valid digits and are not due to spurioussignals which might be received by the receiving antenna 40, such as forexample, when the transponder 28 on the moving package 22 of FIG. 1 isat considerable distance from the receiving antenna 40 and has not yetmoved into the beam width of the receiving antenna 40. The second testperformed by the interrogator 30 is an examination of the first eightbits stored in the shift register 92 to determine that these first eightbits do form the unique sequence for identifying the presence of atransponder 28 as well as establishing that the 40 bits are arranged inthe shift register 92 such that the first bit of the message word ispositioned at one end of the shift register 92 while the last bit of themessage word is positioned at the opposite end of the shift register 92.

The first test, namely the random digit test, is performed in thefollowing manner. In response to the clocking of the shift register 92by the clock pulse signal on line 90,. the shift register 92 becomesfilled with successive samples of the signal on line 86 and, thereafterthe first bits to be stored in the shift register 92 are cast off vialine 94 into a digital comparator 96 which compares the successive logicstates of signals appearing on line 94 with the logic states of signalsappearing on line 86. Since the length of the shift register 92 is equalto the length of a message word, the logic state of a signal sample online 86 should be equal to the logic state of the corresponding signalsample on line 94 since that signal sample appeared exactly 40 bits, or1 message word length, earlier on line 86. The digital comparator 96provides an output signal on line 98 having a logic state of O inresponse to an equality of logic states on the lines 86 and 94, namely,a O and a O or a l and a l; and the digital comparator 96 provides asignal on line 98 having a logic state of l in response to differinglogic states appearing on lines 86 and 94, namely, a O and a l or a land a O.

A counter 100 counts the pulses on line 84, these pulses being appliedto the clock input terminal of the counter 100. The counter 100 countsto a count of 40 and remains at a count of 40 until it is reset to acount of by a pulse signal on line 98 having a logic state of l; thecounter 100 may also be reset at any time prior to its reaching a countof 40 by the signal on line 98. When the counter 100 reaches a count of40 it provides a positive pulse to AND gate 102. The counter 100 canreach the count of 40 only when a total of 40 successive bits on line 86agree with the preceding set of 40 successive bits which occur on line94, since, in the event that any one of these bits fails to agree withthe corresponding bit, the digital comparator 96 provides a logic stateof l on line 98 which resets the counter 100 to 0.

A format detector 104 is responsive to the first 8 bits of the messageword stored in the shift register 92 and, if but only if these first 8bits have the proper values and are in the proper sequence or format,the format detector 104 applies a positive pulse to the AND gate 102.The format detector 104 may comprise, by way of example, a multipleinput AND gate in which certain ones of its inputs are inverted. In thepreferred embodiment of the invention, the first 8 bits are arranged inthe sequence 01 l l l 100 in which case the first, seventh and eighthinputs to the AND gate are preceded by inverters so that when all of thebits are correct, 8 positive pulses appear at the inputs to the AND gatewith the result that a positive pulse appears at the output of the ANDgate.

In response to the positive pulse signals of the counter 100 and theformat detector 104, the AND gate 102 applies a positive pulse on line106 to multivibrator 108. The multivibrator 108 is a monostablemultivibrator providing a negative going pulse at the 6 output and apositive going pulse on the Q output, these pulses having a pulse widthwhich is variable and is preset in a well known manner by means of acontrol 110 connecting with the multivibrator 108. For example, thecontrol 110 may adjust the value of a resistance within themultivibrator 108 to effect a change in the pulse width. The 6 output online 112 normally presents a voltage of a high value to the AND gate 88so that the aforrnentioned pulses on line appear in response to pulseson line 84. In response to the signal on line 106 the voltage on line112 assumes a low value which terminates the presence of clock pulses online 90 during the duration of the negative pulse on line 112. As hasalready been noted, the presence ofa pulse on line 106 indicates thatall digits of a message word are properly positioned within the shiftregister 92 and, accordingly, the cessation of clock pulses on line 90permits the message word to remain stored in the shift register 92 foran interval of time as has been set by the control 110. Upon thetermination of the negative pulse on line 112, clock pulses again appearon line 90 and the foregoing procedure is repeated to admit a newmessage word into the shift register 92. With reference to FIG. 1, ifthe transponder 28 has not moved away from the receiving antenna 40,then the new message word will be the same as the preceding messageword. However, if the transponder 28 has moved away from the receivingantenna 40, and the transponder 28 on the next package 22 has moved infront of the receiving antenna 40, then the new message word to bestored in the shift register 92 may differ from the preceding messageword stored in the shift register 92.

The display 32, seen in both FIGS. 1 and 2, comprises a decoder 114 anda display panel 116. The decoder 114, in response to a READ signal online 117 from the Q output of the multivibrator 108, accepts the last 32bits on 32 parallel conductors indicated by line 118 and decodes thisdigital signal in a well known manner to activate the alphanumericsymbols on the display panel 116 as represented by the various bits online 118 so that the message stored within the shift register 92 appearson the display panel 116. The message displayed on the panel 116 isupdated upon successive applications of the READ signal to the decoder114. The electrical conductors represented by lines 117 and 118 arecontained within the electrical cable 33 of FIG. 1. It is also readilyapparent that, with reference to FIG. 2, the display 32 could bereplaced by a computer (not shown) with the data on the parallelconductors of line 118 being entered into a buffer storage register ofthe computer in response to the READ signal on line 117.

While the digital comparator 96 and the counter have been utilized inthe preferred embodiment for testing the contents of the shift register92, it is understood that other well known methods may also be utilized,particularly the well-known parity check in which one, two or more bitsof the message word may be reserved for parity checks.

Referring now to FIG. 3, there is shown a view, partially diagrammatic,of the transponder 28. A series of five dipole antenna elements 120,some of which are further identified by suffixes 120A-E, are spacedapart from an electrically conducting reflecting plate 122 by a slab 124of low loss insulating material to be described hereinafter. The dipoleelements 120 may be conveniently formed by utilizing a slab 124 which isinitially copper clad, and then etching away the excess copper to leavecopper strips substantially as shown in the drawings which serve as thedipole elements 120. Diodes 126, some of which are further identified bysuffixes 126A-E, interconnect the respective portions of each dipoleelement 120. Conducting wires 128, preferably thin wires having arelatively large inductance such as No. 40 gauge wire, pass through theslab 124 for connection with the anode and cathode terminals of thediodes 126. The wires 128 are interconnected by metallic strips 130 tobe described in further detail with reference to FIG. 4, to provideelectrical connection between the terminals of the diodes 126 and thecorresponding portions of the dipole elements 120 so that the dipoleelements 120A-D are connected in series. Thus, for example, the anode ofdiode 126A is connected to the cathode of diode 126B. The anode of diode126D is grounded at the reflecting plate 122 while the cathode of thediode 126A connects via diode 132 with capacitor 134. One terminal ofcapacitor 134 is grounded at ground 136 to complete a series circuitwith the dipole elements lA-D.

The reflecting plate 122 serves as a shield in the event that thetransponder 28 is mounted on a metallic object and thereby preserves theradiation characteristics of the array of dipole elements 120. Thedipole elements 120AE are spaced from the reflecting plate 122 by adistance of one-quarter of the wavelength of the X-band radiation withinthe material of the slab 124 to maximize the intensity of the electricfield incident upon the dipole elements l20A-E. In the event that thereflecting plate 122 is not required for shielding purposes, as when thetransponder 28 is mounted on a nonconducting object, the reflectingplate 122 may be dispensed with and the slab 124 need be thick enoughonly to provide sufficient strength for supporting the dipole elementsl20A-E in their relative positions. For example, the dipole elementsl20A-E could be mounted within an automobile window in which case theslab 124 would be provided for by the glass of the window. In thoseinstances where the reflecting plate 122 is utilized, it is desirable tominimize the required depth of the slab 124 to minimize the thickness ofthe transponder 28, this being accomplished by utilizing a material ofhigh dielectric constant. For example, in the preferred embodiment theslab 124 has been made of a ceramic material formed of a two phasemixture of Zn-,Ti0,, and Ti0 which provides a dielectric constant ofapproximately 26. Such a ceramic material is described more fully in apatent application entitled Microwave Dielectrics" by Dennis W. Ready etal., filed on Apr. l, l97l and having Ser. No. 130,356. Alternatively, alow dielectric material such as Teflon (polytetrafluoroethylene) may beutilized, but the thickness of the transponder 28 will be greater. Inview of the novel feature of the combination of the dipole elements 120and the diodes 126 such that a single antenna structure performs thedual functions of receiving incident radiation and rectifying it, thestructure has been referred to as a rectenna."

Due to the connections of the diodes 126 to each of the dipole elements120, the current waveform induced in the wires 128 has the form of arectified sine wave. The direct component (DC) of this current waveformis extracted by means ofa low pass filter which attenuates thealternating components of the signal, the low pass filter being composedof the inductances of the wires 128 and the capacitances between themetallic strips 130 and the reflecting plate 122.

This low pass filter is better seen by referring to FIGS. 3, 4 and 5.FIG. 4 is an isometric view of the back side of the transponder 28showing the metallic strips 130 separated from the reflecting plate 122by a thin film 138 ofinsulating material, for example Mylar film,available as a pressure sensitive adhesive tape which is readily appliedto the back side of the reflecting plate 122. Similarly, the metallicstrips 130 can be formed by utilizing commercially available metallizedpressure sensitive adhesive tape which is readily applied over the film138. As seen in FIG. 4, the metallic strips 130 interconnect the wires128 of which the ends are visible in FIG. 4. The widths of the metallicstrips 130 may be selected to provide the desired capacitance betweemthe metallic strips and the reflecting plate 122. The metallic strips130 have been shown in the Figure as being relatively narrow in order tobetter indicate the con nections between the wires 128, however, theymay be widened and also elongated beyond their points of contact withthe wires 128 so that a substantial region of the back side of thetransponder 28 may be utilized to provide capacity for the low passfilter.

FIG. 5 shows a schematic representation of the low pass filter, hereidentified by numeral 140, in which the inductances of the wires 128 arerepresented by inductors 142 and the capacitances between the metallicstrips 130 and the reflecting plate 122 is represented by capacitors144. Also shown connected to the filter 140 is the diode 132 andcapacitor 134 described earlier with reference to FIG. 3. The dipoleantenna elements 120 are indicated schematically by heavy linesconnecting with the terminals of the diodes 126. The inter connection ofthe anode of diode 126D via inductor 142 to ground 136 is accomplished,as seen in FIG. 4, by removing a portion of the film 138 to expose aregion 146 of the reflecting plate 122 for making electrical contactbetween the metallic strip 130 and the reflecting plate 122.

In the preferred embodiment the diodes 126 are microwave diodes, type1N82A, and the serial interconnection of the four diodes I26A-D provides5 volts at a distance of 30 feet from the interrogator 30 with thetransponder 28 being illuminated with a peak power intensity 0f 10milliwatts per square centimeter. A total of 15 volts can be obtainedfrom the serially connected diodes l26A-D at a distance of 5 feet. Thetransmitting antenna 38 of the interrogator 30 has a 26 db (decibels)gain and the receiving antenna 40 has a 20 db gain. The beam widthprovided by the array of dipole elements 120 is relatively broad, thebeam having the form of a cone of beam width with the axis of the conebeing perpendicular to the plane of the reflecting plate 122. It isdesirable to maintain these spacings between neighboring dipoleelements, such as the elements A-B, greater than one-half wave length toreduce mutual coupling between these dipole elements,

a spacing of approximately one wave length having been successfully usedin the preferred embodiment. The radiators of the dipole elements 120may be either of rectangular shape or, as seen in FIG. 3, be providedwith a slight taper for an improved impedance match. The diode leads,themselves, of the diodes 126 have been found to function effectively asdipole elements 120. The wires 128 are perpendicular to the reflectingplate 122 so as to minimize interaction direction with theelectromagnetic fields incident upon the array of dipole elements 120.The interrogator 30 of the preferred embodiment has utilized a 2kilowatt peak power transmitter.

As shown in FIG. 3, the transponder 28 further comprises an oscillator148 composed of a tunnel diode 150, an inductor 152 formed of afractional turn of wire and having an inductance of substantially lessthan one microhenry, a capacitor 154 having a value of 0.02 microfaradsfor storing energy provided by an incident pulse of electromagneticradiation, and a dipole antenna 156 composed of a pair of radiators 156Aand 1568. The dipole antenna 156 has an overall length, in the preferredembodiment, of approximately 2 inches which is substantially smallerthan the wave length, 25 centimeters, of radiation radiated by theantenna 156. Thus, the dipole antenna 156 approximates a point sourcewith an almost omnidirectional radiation pattern. Each of the radiators156A-B may have a tapered form as shown in the Figure, or other form inwhich the outer extremity is larger than the inner extremity to providea more favorable impedance match. A wire 158 is mounted perpendicularlyto the reflecting plate 122 and passes through the slab 124 to connectwith the junction of the inductor 152 and the capacitor 154. A secondwire 160 is positioned perpendicularly to the reflecting plate 122 andgrounded thereto, and passes through the slab 124 to contact thejunction of the radiator 156A, the tunnel diode 150 and the capacitor154. A source of power to be described hereinafter is applied to theoscillator 148 such that the power supply voltage is impressed betweenthe wire 158 and ground 136.

Referring now to FIGS. 3, 6, 7 and 8, power for the oscillator 148 isprovided by the diode 126E in a manner analogous to the aforementionedoperation of the serially connected diodes 126AD. As seen in FIG. 6, thediode 126E comprises a well known stray capacitance and resistancerepresented by dotted lines and indicated by the numerals 162 and 164respectively. The power supply portion of the circuit of FIG. 6 isindicated by numeral 166 and is seen comprising, in addition to thediode 126E, the dipole antenna element 12013, and filtering elementsanalogous to those of FIG. 5. The filtering elements of FIG. 6 are apair of inductors 168 representlng respectively the inductance of eachof the wires 128 (of FIG. 3) connecting with the diode 126E, a capacitor170 representing the capacitance between the metallic strip 172 (seenalso in FIG. 4) and the reflecting plate 122, and inductors 174 and 176representing respectively the inductances of the wires 158 and 160 (ofFIG. 3). In addition, FIG. 6 shows an interconnection along line 178 toan insulated gate field effect transistor, hereinafter referred to asFET 180, to be described hereinafter.

The oscillator 148 is a relaxation type oscillator utilizing a nonlinearactive element such as the tunnel diode 150 and the inductor 152. Forexample, a tunnel diode type IN3713 may be utilized. A typicalcurrentvoltage graph for a tunnel diode is shown in FIG. 7 wherein V1represents the voltage to which the capacitor 154 is initially chargedby the power supply 166 in response to a pulse of radiation incidentupon the dipole element 128E. The operation of the oscillator 148 is asfollows: After the charging of the capacitor 154, current flows from thecapacitor 154 through the inductor 152 and the tunnel diode 150 therebyslowly discharging the capacitor 154 and reducing its voltage to a valueof V2 whereupon well known relaxation oscillations begin and continueduring a further discharging of the capacitor 154 until the voltageacross the tunnel diode 150 reaches a value of V3. ,7

The upper graph 184 of FIG. 8 represents the voltage across the tunneldiode 150 as a function of time. The lower graph 186 shows two pulses ofradiant energy, 188 and 190, which represent respectively the pulse ofradiant energy incident upon the dipole element E of FIGS. 3 and 6, andthe pulse of radiation emanating from the radiators 156A-B of FIGS. 3and 6. The pulse 188 which energizes the oscillator 148 is seen having aduration of approximately 1 microsecond while the pulse 190 generated bythe oscillator 148 is seen having a duration of approximately 5microseconds. There is a time delay of approximately 5 to 30microseconds be tween the leading edge of the pulse 188 and the leadingedge of the pulse 190, this delay being the interval of time duringwhich the voltage across the tunnel diode drops from a value of V1 tothe value of V2. The value of V1 depends on the intensity of theradiation incident upon the dipole antenna element 120E, while the valueof V2 is dependent on the characteristics of the tunnel diode 150. Thusthe interval of time during which the voltage across the tunnel diode150 drops from the value of V1 to the value V2 similarly depends on theintensity of the incident radiation, or, with reference to FIG. 1, onthe distance between the interrogator 30 and the transponder 28. Thisinterval between the leading edges of the pulses 188 and 190 is the sameinterval of time which was mentioned earlier with reference to theinterrogator 30 of FIG. 2 with respect to the delay provided by themultivibrator 70.

As seen in the graph 184 of FIG. 8, the voltage across the tunnel diode150 continues to drop towards zero even after the termination of theoscillations at the value of V3. The stray resistance 164 of the diode126E provides via the wires 128, 158 and a direct path to ground 136which discharges the capacitor 154 so that there is essentially noresidual charge left on the capacitor 154 when the next pulse 188 ofgraph 186 arrives. This insures that only one pulse is produced by theoscillator 148 for each pulse 188 of radiation incident upon thetransponder 28.

Returning to FIG. 3, pulses of energy appearing on line 192 in responseto the radiation incident upon the dipole elements 120A-D are appliedvia an inverting amplifier 194 along line 196 to a counter 198 and aflip-flop 208. It is noted that as the transponder 28 is firstilluminated with incident radiation, the pulses appearing on line 192are of a relatively low value until such time as the capacitor 134becomes charged to its full voltage. Since the capacitor 134 is arelatively large size, 0.1 microfarads being utilized in the preferredembodiment, a number of pulses of reduced amplitude appear on line 192as the capacitor 134 is charged to its full voltage and, thereafter, thepulses on line 192 have the requisite magnitude of from to 15 volts(depending on the intensity of the incident radiation as was notedhereinbefore this being adequate to operate the counter 198 and theflip-flop 200.

A diode matrix array 202 having a well known form comprises a set of rowconductors 204 and a set of column conductors 206 having cross points208 some of which are interconnected by diode 210. Only a portion of thediode array 202 is shown in FIG. 3. Any number of row conductors 204 andcolumn conductors 206 may be utilized, the number depending on theamount of storage capacity desired for the storage of the message word.In the preferred embodiment, four row conductors 204 are utilized and 10column conductors 206 are utilized.'Accordingly, the counter 198 of thepreferred embodiment is a 2 bit counter for activating the four rowconductors 204 by means of a decoder 212. In response to each of thecounts from the counter 198 the decoder 212, in a well known fashion,energizes successive ones of the row conductors 204. In a similar mannera counter 214 and a decoder 216 having a capacity of 10 energize the 10column conductors. The counters 198 and 214, the decoders 212 and 216,as well as the flip-flop 200 and the inverting amplifier 194 are eachenergized with power received from the energy storage capacitor 134between terminal P and ground 136. The counter 214 is activated by thelast bit of the counter 198 so that the count of counter 214 is alteredonce each time the counter 198 reaches a count of 4. Both the counters198 and 214 reset themselves to 0 after reaching their maximum count Forease of reference, the row conductors 204 will be further identified bysuffixes such as the row conductor 204A and the column conductors willbe further identified by suffixes such as the column conductor 206A.Each of the row conductors 204 is energized via resistors 218, some ofwhich are further identified by suffixes such as resistor 218A, whichconnect with respective terminals of the decoder 212. Each row conductor204 connects with a diode 220, some of which are further identified bysuffixes such as diode 220A. The anodes of the four diodes 220 connectvia line 222 to flipflop 200, the signal on line 222 serving as the SETsignal for the flip-flop 200. A resistor 224 is connected between line222 and terminal P of capacitor 134.

The 0 output of flip-flop 200 is connected to the gate terminal of theFET 180 such that when the flip-flop has been set by a signal on line222, a voltage of sufficient magnitude is applied to the gate of the FET180 to place it in a state of conduction. When the flip-flop 200 isreset by the signal on line 196, the voltage at the Q terminal of theflip-flop 200 is of a sufficiently low value such that the FET 180 is ina state of nonconduction. For example, during a reset condition theflip-flop 200 provides a voltage at the Q terminal on the order of a fewtenths of a volt which is sufficient to maintain the FET 180 in a stateof nonconduction. And during a set condition, the flip-flop 200 providesat its 0 terminal a voltage of 5 volts or more (depending on themagnitude of the voltage stored by capacitor 134) which is sufficient tomaintain the FET 180 in a state of condution. By way ofexample ofcomponents that may be utilized, the preferred embodiment utilizes asFET 180 an FET of the Radio Corporation of America (RCA) having a partnumber CD4016E. The counter 214 and decoder 216 are sold as a singleunit by RCA having the part number CD40] 75.

In operation, therefore, the transponder 28 provides a digitally codedtrain of pulses in response to an incident radiant energy pulse train inthe following manner, When the counter 198 registers a count of l, thedecoder 212 in response to the count of 1 applies a logic state ofO tothe resistor 218A, the logic state of0 being a voltage of value lessthan 1 volt. The row conductor 204A may or may not be at this lowvoltage depending upon interconnections with the column conductors 206as will be described hereinafter. When the counter 198 attains the countof 2, the decoder 212 applies a logic state ofO to the resistor 218B andrestores the row conductor 204A to a logic state of I, this being arelatively high voltage of approximately 5 volts or greater. Theprocedure repeats itself for counts of 3 and 4 by the counter 198 sothat at a count of 4 the logic state of 0 is applied to resistor 218D.Since the counter 198 is modulo-4, upon receipt of the next pulse online 196, the counter 198 reverts to the state representing a count of lin which case the logic state of0 is again applied to the resistor 218A.

When the counter 198 recycles from the count of 4 to the count of 1, itapplies a pulse along line 226 to the counter 214 which, in responsethereto, increases its count by count of l with the result that thedecoder 216 applies a signal to the next column conductor 206 in amanner analogous to the operation of the decoder 212. It should be notedthat the decoder 216 applies logic states of l to the column conductor206 corresponding to the count of the counter 214 while the decoder 212applies logic states of 0 to the resistor 218 corresponding to the countof the counter 198.

For example, assuming that at some point in time the column conductor206C is energized with a high voltage while a low voltage is applied tothe resistor 2188 There is no connection between the column conductor206C and the row conductor 2048 at their cross point 208 so that a lowvoltage is applied to the cathode of diode 220B. The value of a resistor218 is much smaller than the value of the resistor 224, for example,values of 10,000 and 100,000 ohms being used respectively in thepreferred embodiment and being indicated in FIG, 3 by the legends 10Kand K Thus the application of the low voltage to the cathode of thediode 2208 results in a low voltage appearing on line 222 which placesthe flip-flop 220 in a SET condition.

In response to the next pulse to appear on line 196, the counter 198advances to a count of 3, but the low voltage remains on the line 222since there is no connection between the column conductor 206C and thenext row conductor. However when the counter 198 reaches a count of 4and the low voltage is applied to resistor 218D, it is noted that thereis a diode 210 at the cross point 208 for providing a conducting pathbetween the column conductors 206C and the row conductor 204D with theresult that the row conductor 204D remains at the high voltage (exceptfor a fractional volt drop across the diode 210) with a voltage dropappearing across the resistor 218D, this voltage drop being equal to thedifference in voltages between the high voltage on line 204D and the lowvoltage being applied to the terminal of the resistor 218D next to thedecoder 212. Thus it is seen that a high voltage is applied to thecathode of each of the diodes 220 with the result that a high voltageappears on line 222 thus terminating the SET signal to the flip-flop200.

Continuing now, in response to the next pulse on line 196, the counter198 transmits a pulse along line 226 and reverts to a count of I. Inresponse to the pulse on line 226 the counter 214 advances one count,and in response thereto the decoder 216 energizes the next columnconductor 206D with a high voltage while applying a low voltage to theother column conductors 206. The decoder 212 in response to the count of1 at the counter 198 applies a low voltage to the resistor 218A while ahigh voltage is being applied to each of the other resistors 218. Sincethere is no connection at the cross point 208 between the columnconductor 206D and the row conductor 204A, a low voltage appears at thecathode of the diode 220A while high voltages appear at the cathodes ofthe other diodes 220 with the result that the low voltage appears online 222 for setting the flip-flop 200. In response to the next pulse toappear on line 196, the decoder 212 applies the low voltage to theresistor 218B with the result that current flows through the diode 210at the cross point 208 between the column conductor 206D and the rowconductor 204B. As a result of the current flow from the decoder 216along the column conductor 206D through the diode 210 and the resistor2188, the row conductor 2048 is at a high voltage (as is each of theother row conductors 204) with the result that a high voltage appears onthe line 222 which terminates the setting of the flip-flop 200.

It is noted that a RESET pulse appears on line 196, as has beenmentioned hereinbefore, for each pulse of radiant energy incident uponthe diode antenna elements 120. However, with respect to the signal online 222, a SET signal may, or may not, appear in response to each pulseon line 196 depending on the positioning of diodes at the cross points208. If this SET signal does appear, it appears a fractlon ofamicrosecond after the RESET signal and is retained throughout theinterpulse period between successive ones of the pulses 196 so that theflip-flop 200 is then placed in a SET condition. As a practical matterin the selection of the coss points 208 which are to be provided withdiodes 210, it is convenient to purchase commercially a diode matrixarray and then to apply sufficiently large currents simultaneously to arow and a column conductor to burn out the diode at their cross point.By burning out the unwanted diodes the diode matrix array 202 attainsits desired format.

The FET 180 of FIG. 3 shorts out the capacitor 154 in response to eachsetting of the flip-flop 200. As seen in FIG. 8, there is anapproximately 530 microsecond interval between the arrival of a pulse online 196 and the oscillation by the oscillator 148 so that there isample time for the FET 180 to short out the capacitor 154 prior to theinception of an oscillation. Thus it is seen that for each pulse on line196 an oscillation is produced by the oscillator 148 unless a SET signalappears on line 222. As a result, in response to a continuous successionof pulses of radiant energy incident upon the dipole antenna elements120, there is an intermittent succession of pulses of radiant energytransmitted by the dipole antenna 156, the presence and absence oftransmissions by the dipole antenna 156 being governed by the positionsof diodes 210 in the diode matrix array 202. The occurrences and theabsences of transmissions by the dipole antenna 156 constitutes thedigital coding of the pulse train signal provided by the transponder 28in response to the succession of radiant energy pulses incidentthereupon. The extraction of information from the digitally coded signalhas been described hereinbefore with reference to FIG. 2.

The digital electronics portion of the transponder 28, namely, the diode132, the capacitor 134, the inverting amplifier 194, the FET 180, theflip-flop 200, and the counters, decoders and diode matrix array withits interconnections to the flip-flop 200 may be fabricated utilizing alarge scale integration (LSI) package of sufficiently small size to beplaced on the back side of the transponder 28 as seen by the emplacementof package 228 of the LSI circuitry. Connections from the package 228are provided by metallic strips 230, 232 and 234, the metallic strip 232being the interconnection between diode 126A and diode 132 of FIG. 3,the metallic strip 234 being the interconnection shown schematically byline 178 in FIG. 3, and the metallic strip 236 being a groundconnection, such as the grounding of one terminal of a capacitor 134, toan exposed region 146 of the reflecting plate 122. It is also noted thatthe average power dissipated by the circuitry of the tran sponder 28 isapproximately microwatts, and the average power transmitted by thedipole antenna 156 is 2 microwatts in the preferred embodiment of theinvention.

Referring now to FIG. 9 there is shown a detailed schematic diagram ofthe counter 198 and the decoder 212 of FIG. 3. The counter 198 comprisestwo flipflops 236 and 238, each being a D-edge flip-flop. In thepreferred embodiment the flip-flops 236 and 238 are within an integratedcircuit by RCA type No. CD40l3E. The decoder 212 comprises four multipleinput AND gates 240A-D to which the two flip-flops 236 and 238 areinterconnected as shown in the drawing. Also shown in the drawing is aNAND gate which serves as the inverting amplifier 194 as well as asecond NAND gate 242 which again inverts the output of the invertingamplifier 1 94 and applies this to the four NAND gates 240A-D. Theflip-flop 200 comprises a pair of NAND gates interconnected as shown inthe drawing. Also seen in the drawing are the diodes 132 (type 1N4606being utilized in the preferred embodiment), capacitor 134, resistors218A-D and the row conductors 204 for the diode array 202, all of whichhave been described with reference to FIG. 3. In the preferredembodiment, the inverting amplifier 194, the NAND gate 242 and the NANDgates of the flip-flop 200 are included within an integrated circuit,RCA type CD401 1E. The four NAND gates 240A-D are integrated circuits,RCA type CD4012E. The diode array 202 is an integrated circuit, HarrisIntertype diode array, type RMl04l0-5.

Referring now to FIG. 10 there is shown a partial view of an alternativeembodiment of the transponder 28 of FIG. 3, here designated by thenumeral 244. The transponder 244 comprises the same slab 124, reflectingplate 122, the dipole antenna 156 of which only the radiator 156A isseen in the Figure, and the diodes 126 of which only diodes 126A and126B are seen in the Figure. However. a double leaved spiral antennaelement 246 having leaves 246A-B is utilized in lieu of the dipoleelements of FIG. 3. The spiral antenna element 246 is responsive to bothcircularly and linearly polarized radiation so that the transponder 244may be mounted on an object, such as the packages 22 of FIG. 1, in avariety of orientations while still being responsive to the radiationtransmitted by the interrogator 30.

The diodes 126 are connected by wires 128 in the same manner as shownfor the transponder 28 of FIG. 3. One leaf 246A connects with the anodeof the diode 126 while the other leaf 246B connects with thecathode ofthe diode 126. The spiral antenna elements 246 may be fabricated in amanner similar to that utilized for the transponder 28 of FIG. 3,namely, by bonding a copper sheet to the front face of the slab 124 andetching away the unwanted portions of the copper.

Referring now to FIG. Ill, there is shown an alternative embodiment ofthe diode array 202 of FIG. 3, here designated by numeral 248. The diodearray 248 utilizes the same row conductors 204, the same columnconductors 206, and the same diodes 210 as does the diode array 202.However, the diode array 248 is provided with additional leads 250whereby the anode of each diode 210 can be connected by a switch 252,external to the diode array 248, to a column conductor 206 to provide aconducting path at a cross point 208 via the diode 210 between thecolumn conductor 206 and the row conductor 204. By simply opening andclosing selected ones of the switches 252, any desired message can bestored in the diode array 248. In this respect, the diode array 248 canbe regarded as a readonly memory in which the stored message can bereadily altered as desired. This is particularly useful in the situationwhere the transponder 28 and interrogator 30 are to be utilized in anautomobile rental system wherein an automobile passing through theentrance of a parking lot must identify its serial number, mileage,destination and similar matters.

Referring now to FIG. 12, there is shown a hospital nurses station 254in which containers of medicine 256 and 258 are placed on a shelf 260within easy reach of a nurse 262. The nurse 262 is seated at a desk 264on which is placed an encoder unit 266 which encodes the stored messagewithin the diode array 202 of the transponder 28, seen nested within theencoder unit 266. Here, the row conductors 204 and the column conductors206 of FIG. 3 have been extended towards the edges of the transponder 28so that electrical contact can be made with selected ones of the rowconductors 204 and the column conductors 206 for applying a surgecurrents which burn out selected diodes 210 for encoding the message tobe stored within the diode array 202. The encoder unit 266 comprises akeyboard 268, a well known logic unit 270 which selects the particularrow conductor 204 and column conductor 206 which correspond to aparticular key of the keyboard, and a current supply 272 for injectingthe surge current between the selected row conductor 204 and columnconductor 206. As seen in the Figure, the current supply 272 ispartially cut away to disclose probes 274 which contact the transponder28 at the extremeties of the row conductors 204. A similar set ofprobes, not seen in the Figure, makes contact with the column conductors206 of the transponder 28.

In operation, therefore, a pair of probes 274 one for a row conductor204 and one for a column conductor 206 are energized with current fromthe current supply 272 in accordance with signals provided by the logicunit 270 in response to the depressing of a key on the keyboard 268. Thetransponder 28 after having been encoded with information such as apatients name, the type of medicine, and the times of administering themedication, is then affixed to a medicine container such as the medicinecontainer 258. The transponder 28 may have an adhesive backing tofacilitate attachment to the medicine container 258 or, may simply beheld in place by pressure sensitive adhesive tape. The same procedurewould be utilized in the laboratory of the hospital in the preparationof blood transfusions in which case the encoded message would includedata such as the blood type. Prior to the administering of the medicineor the blood transfusion, the container would be placed in front of aninterrogator, such as the interrogator 30 of FIG. 1, and a display wouldverify the contents of the container while an optional connection with acomputer would provide an entry for inventory control to indicate theamount of medicine or blood re maining in stock.

It is understood that the above described embodiments of the inventionare illustrative only and that modifications thereof will occur to thoseskilled in the art. Accordingly, it is desired that this invention isnot to be limited to the embodiments disclosed herein but is to belimited only as defined by the appended claims.

What is claimed is:

1. In combination:

means for extracting power from an incident radiant signal comprising asuccession of pulses; and

means energized by said extracted power for generating a train of pulsescoded in a digital format, said coded pulse generating means comprisingmeans for gating pulses of said train of pulses in synchronism with theindividual pulses of said succession of pulses of said radiant signal.

2. The combination according to claim 1 further comprising meansresponsive to said coded pulse train for displaying information providedby said coded pulse train.

3. The combination according to claim 1 further comprising:

a source of said radiant signal, said radiant signal comprising asuccession of pulses of radiant energy;

means for receiving said coded pulse train; and

means synchronized to transmissions of said radiant energy pulses forcorrelating at least a portion of said coded pulse train with areference code.

4. In combination:

means responsive to an incident radiant signal for extracting power fromsaid radiant signal;

means energized by said extracted power for generat ing a train ofpulses coded in a digital format, said power extracting meanscomprising:

a plurality of antenna elements;

a plurality of serially connected diodes, each of said diodes beingcoupled to respective ones of said antenna elements; and

means for storing power extracted by said diodes from a signal incidentupon said antenna elements.

5. The combination according to claim 1 wherein said generating meansincludes means for varying said code.

6. The combination according to claim 1 wherein said radiant signalcomprises a train of first pulses, and said coded signal comprises atrain of second pulses, the occurrences of predetermined ones of saidsecond pulses being deleted from said train of second pulses inaccordance with said code.

7. In combination:

a radiating element;

diode means coupled to said radiating element for extracting power fromradiation incident upon said radiating element;

counting means energized from said power for counting successivepulsations in said power;

an oscillator circuit for providing a radiant energy signal; and

logic means responsive to the count of said counter for activating saidoscillator circuit.

8. The combination according to, claim 7 wherein said logic meanscomprises a matrix conductor array having preselected cross pointscoupled during one condition of relative polarities of conductors at across point and decoupled during an alternate condition of relativepolarities of conductors at said cross point, said conductors beingenergized respectively by said counting means in accordance with thecount of said counter.

9. The combination according to claim 8 wherein said oscillator circuitcomprises:

an energy storage unit;

a reactive impedance element; and

a nonlinear active element cooperating with said reactive impedanceelement to provide a relaxation type oscillation, the energy for saidrelaxation oscillations being supplied by said energy storage unit, saidrelaxation oscillations being delayed in time from a charging of saidenergy storage unit until current drained via said nonlinear activeelement has reduced the voltage of said energy storage unit to a valuesuitable for initiating said relaxation oscillations.

10. The combination according to claim 9 wherein said counting meanscomprises a first counter and a second counter, said first counter andsaid second counter each having a decoder for decoding the counts ofsaid first counter and said second counter, each of said decodersenergizing respective groups of conductors of said matrix conductorarray.

11. The combination according to claim 10 wherein said nonlinear activeelement is a tunnel diode.

12. The combination'according to claim 11 wherein said oscillatorcircuit further comprises an antenna for transmitting said radiantenergy signal of said oscillator circuit at a frequency different fromthe frequency of said radiation incident upon said radiating element.

13. A message communication system comprising:

means for generating an interrogating signal, said interro-gating signalcomprising a succession of pulses;

means for applying a stored message to an object;

means energized solely by power from said interrogating signal fortransmitting said stored message from said object, said transmittingmeans comprising means for gating said transmitted message insynchronism with individual pulses of said succession of pulses of saidinterrogating signal to provide gated portions of said transmittedmessage which correspond to respective ones of said pulses of saidsuccession of pulses; and

means for receiving said transmitted message, sald receiving meansincluding means synchronized with individual pulses of said successionof pulses of said interrogatang signal for performing a correlation onsaid received message.

14. A message communication system comprising:

means for generating an interrogating signal, said interro-gating signalcomprising a succession of pulses;

means for applying a stored message to an object;

means energized solely by power from said interrogating signal fortransmitting said stored message from said object, said transmittingmeans comprising means for synchronizing successive portions of saidtransmitted message with said interrogating signal; and

means for receiving said transmitted message.

15. The system according to claim 14 wherein said message applying meansincludes a memory storage array in which bits of data are individuallystored.

16. The system according to claim 15 wherein said bits of stored datamay be individually varied.

17. The system according to claim 16 wherein said message applying meanscomprises means attached to said object for varying said bits of storeddata.

18. The system according to claim 17 wherein said transmitting meanscomprises means for extracting energy from said interrogating signal andstoring such energy, and means for reradiating a portion of such energyas a signal modulated with said bits of data to ac complish saidtransmission of said stored message.

19. A transponder for communicating a message comprising:

means responsive to a succession of pulses of incident radiant energyfor providing a unidirectional voltage therefrom;

means for storing energy of a direct component of said unidirectionalvoltage;

means for reradiating a portion of said stored energy;

means for counting pulses of said succession of pulses of incidentenergy;

means for storing a message, said storing means including meansresponsive to the count of said counting means for extractinginformation from said stored message; and

means for modulating a signal of said reradiated energy with saidinformation of said stored signal.

20. A transponder for communicating a message comprising:

means responsive to incident radiant energy for providing aunidirectional voltage therefrom;

means for storing energy of a different component of said unidirectionalvoltage;

means for reradiating a portion of said stored energy;

means for storing a message;

means for modulating a signal of said reradiated energy with informationof said stored signal, and wherein said unidirectional voltage means comprises: an array of radiators for intercepting said incident radiationand providing an electric field therefrom;

means for providing a ground plane, said radiators being spaced apartfrom each other and being uniformly spaced from said ground plane;

conducting elements serially interconnecting pairs of said radiators,said conducting elements extending from said radiators towards saidground plane, a first part of individual ones of said conductingelements being parallel to said ground plane to provide predeterminedvalues of capacitance for filtering energy of said electric field; and

rectifier means coupled between pairs of said radiators and responsiveto said electric field for providing said unidirectional voltage.

21. The transponder according to claim wherein a second part ofindividual ones of said conducting elements of said unidirectionalvoltage means are configured for providing inductances, saidcapacitances and said inductances serving as a filter for extractingsaid direct component of said unidirectional voltage.

22. The transponder according to claim 21 wherein said message storagemeans comprises a first and a second set of conductor elements of whicha preselected conductor element of said first set communicates with aplurality of preselected conductor elements of said second set.

23. The transponder according to claim 23 further comprising switchingmeans interconnecting with said message storage means for altering saidcommunications between conductor elements of said first and said secondsets of conductor elements.

24. A transponder for communicating a message comprising:

means responsive to incident radiant energy for providing aunidirectional voltage therefrom, said unidirectional voltage meanscomprising an array of radiators for intercepting said incidentradiation and providing an electric field therefrom, and rectifier meansresponsive to said electric field for providing said unidirectionalvoltage;

means for storing energy of a direct component of said unidirectionalvoltage, said energy storage means including a filter for extractingsaid direct component of said unidirectional voltage;

means for reradiating a portion of said stored energy;

.means for storing a message, said message storage means comprising afirst and a second set of com ductor elements in which a preselectedconductor element of said first set communicates with a plurality ofpreselected conductor elements of said second set, said message storagemeans further comprising means coupled to said first set of conductorelements for counting successive quanta of said incident radiant energy;

means coupled to said reradiating means for modulating a signal of saidreradiated energy with information of said stored message; and

switching means interconnecting with said message storage means foraltering said communications between conductor elements of said firstand said second set of conductor elements.

25. The transponder according to claim 24 wherein said modulating meansis responsive to currents communicated between a pair of said conductorelements of which one element of said pair is of said first set and theother element of said pair is of said second set of conductor elements,different pairs of said conductor elements being activated in accordancewith the count of said counting means.

26. The transponder according to claim 25 wherein said modulating meansmodulates said signal of reradiated energy with the portion of theinformation of said stored signal corresponding to the pair of conductorelements activated by said counting means at each count of said countingmeans.

27. The transponder according to claim 26 wherein said modulation ofsaid signal of said reradiated energy is a pulse code modulation.

: UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3 7 55 9 Dated July 1L 1973 inventor-( George A. WOI'kS 611'. 8.1

It is certified that error appears in the above-identified patent andthat said Letters Patent are hereby corrected as shown below:

Column 1, line ,65, "an" should be and Column 6, line 3, v "Q" should beQ Column 15, line -36, "fract 1 on" should be fraction Column 15, line40, "coss" should be cross Column 21, line 16, C1aim'23 "23" (secondoccurrence) 7 should be "22".

Signed and sealed this 6th day of August 1974.

(SIEAL) Attest:

McCOY M. GIBSON, JR. C. MARSHALL DANN Attesting Officer Commissioner ofPatents ORM PO-10SO (10-69)

1. In combination: means for extracting power from an incident radiantsignal comprising a succession of pulses; and means energized by saidextracted power for generating a train of pulses coded in a digitalformat, said coded pulse generating means comprising means for gatingpulses of said train of pulses in synchronism with the individual pulsesof said succession of pulses of said radiant signal.
 2. The combinationaccording to claim 1 further comprising means responsive to said codedpulse train for displaying information provided by said coded pulsetrain.
 3. The combination according to claim 1 further comprising: asource of said radiant signal, said radiant signal comprising asuccession of pulses of radiant energy; means for receiving said codedpulse train; and means synchronized to transmissions of said radiantenergy pulses for correlating at least a portion of said coded pulsetrain with a reference code.
 4. In combination: means responsive to anincident radiant signal for extracting power from said radiant signal;means energized by said extracted power for generating a train of pulsescoded in a digital format, said power extracting means comprising: aplurality of antenna elements; a plurality of serially connected diodes,each of said diodes being coupled to respective ones of said antennaelements; and means for storing power extracted by said diodes from asignal incident upon said antenna elements.
 5. The combination accordingto claim 1 wherein said generating means includes means for varying saidcode.
 6. The combination according to claim 1 wherein said radiantsignal comprises a train of first pulses, and said coded signalcomprises a train of second pulses, the occurrences of predeterminedones of said second pulses being deleted from said train of secondpulses in accordance with said code.
 7. In combination: a radiatingelement; diode means coupled to said radiating element for extractingpower from radiation incident upon said radiating element; countingmeans energized from said power for counting successive pulsations insaid power; an oscillator circuit for providing a radiant energy signal;and logic means responsive to the count of said counter for activatingsaid oscillator circuit.
 8. The combination according to claim 7 whereinsaid logic means comprises a matrix conductor array having preselectedcross points coupled during one condition of relative polarities ofconductors at a cross point and decoupled during an alternate conditionof relative polarities of conductors at said cross point, saidconductors being energized respectively by said counting means inaccordance with the count of said counter.
 9. The combination accordingto claim 8 wherein said oscillator circuit comprises: an energy storageunit; a reactive impedance element; and a nonlinear active elementcooperating with said reactive impedance element to provide a relaxationtype oscillation, the energy for said relaxation oscillations beingsupplied by said energy storage unit, said relaxation oscillations beingdelayed in time from a charging of said energy storage unit untilcurrent drained via said nonlinear active element has reduced thevoltage of said energy storage unit to a value suitable for initiatingsaid relaxation oscillations.
 10. The combination according to claim 9wherein said counting means comprises a first counter and a secondcounter, said first counter and said second counter each having adecoder for decoding the counts of said first counter and said secondcounter, each of said decoders energizing respective groups ofconductors of said matrix conductor array.
 11. The combination accordingto claim 10 wherein said nonlinear active element is a tunnel diode. 12.The combination according to claim 11 wherein said oscillator circuitfurther comprises an antenna for transmitting said radiant energy signalof said oscillator circuit at a frequency different from the frequencyof said radiation incident upon said radiating element.
 13. A messagecommunication system comprising: means for generating an interrogatingsignal, said interro-gating signal comprising a succession of pulses;means for applying a stored message to an object; means energized solelyby power from said interrogating signal for transmitting said storedmessage from said object, said transmitting means comprising means forgating said transmitted message in synchronism with individual pulses ofsaid succession of pulses of said interrogating signal to provide gatedportions of said transmitted message which correspond to respective onesof said pulses of said succession of pulses; And means for receivingsaid transmitted message, sa1d receiving means including meanssynchronized with individual pulses of said succession of pulses of saidinterrogating signal for performing a correlation on said receivedmessage.
 14. A message communication system comprising: means forgenerating an interrogating signal, said interro-gating signalcomprising a succession of pulses; means for applying a stored messageto an object; means energized solely by power from said interrogatingsignal for transmitting said stored message from said object, saidtransmitting means comprising means for synchronizing successiveportions of said transmitted message with said interrogating signal; andmeans for receiving said transmitted message.
 15. The system accordingto claim 14 wherein said message applying means includes a memorystorage array in which bits of data are individually stored.
 16. Thesystem according to claim 15 wherein said bits of stored data may beindividually varied.
 17. The system according to claim 16 wherein saidmessage applying means comprises means attached to said object forvarying said bits of stored data.
 18. The system according to claim 17wherein said transmitting means comprises means for extracting energyfrom said interrogating signal and storing such energy, and means forreradiating a portion of such energy as a signal modulated with saidbits of data to accomplish said transmission of said stored message. 19.A transponder for communicating a message comprising: means responsiveto a succession of pulses of incident radiant energy for providing aunidirectional voltage therefrom; means for storing energy of a directcomponent of said unidirectional voltage; means for reradiating aportion of said stored energy; means for counting pulses of saidsuccession of pulses of incident energy; means for storing a message,said storing means including means responsive to the count of saidcounting means for extracting information from said stored message; andmeans for modulating a signal of said reradiated energy with saidinformation of said stored signal.
 20. A transponder for communicating amessage comprising: means responsive to incident radiant energy forproviding a unidirectional voltage therefrom; means for storing energyof a different component of said unidirectional voltage; means forreradiating a portion of said stored energy; means for storing amessage; means for modulating a signal of said reradiated energy withinformation of said stored signal, and wherein said unidirectionalvoltage means comprises: an array of radiators for intercepting saidincident radiation and providing an electric field therefrom; means forproviding a ground plane, said radiators being spaced apart from eachother and being uniformly spaced from said ground plane; conductingelements serially interconnecting pairs of said radiators, saidconducting elements extending from said radiators towards said groundplane, a first part of individual ones of said conducting elements beingparallel to said ground plane to provide predetermined values ofcapacitance for filtering energy of said electric field; and rectifiermeans coupled between pairs of said radiators and responsive to saidelectric field for providing said unidirectional voltage.
 21. Thetransponder according to claim 20 wherein a second part of individualones of said conducting elements of said unidirectional voltage meansare configured for providing inductances, said capacitances and saidinductances serving as a filter for extracting said direct component ofsaid unidirectional voltage.
 22. The transponder according to claim 21wherein said message storage means comprises a first and a second set ofconductor elements of which a preselected conductor element of saidfirst set communicates with a plurality of preselected conductorelements of said second set.
 23. The tRansponder according to claim 23further comprising switching means interconnecting with said messagestorage means for altering said communications between conductorelements of said first and said second sets of conductor elements.
 24. Atransponder for communicating a message comprising: means responsive toincident radiant energy for providing a unidirectional voltagetherefrom, said unidirectional voltage means comprising an array ofradiators for intercepting said incident radiation and providing anelectric field therefrom, and rectifier means responsive to saidelectric field for providing said unidirectional voltage; means forstoring energy of a direct component of said unidirectional voltage,said energy storage means including a filter for extracting said directcomponent of said unidirectional voltage; means for reradiating aportion of said stored energy; means for storing a message, said messagestorage means comprising a first and a second set of conductor elementsin which a preselected conductor element of said first set communicateswith a plurality of preselected conductor elements of said second set,said message storage means further comprising means coupled to saidfirst set of conductor elements for counting successive quanta of saidincident radiant energy; means coupled to said reradiating means formodulating a signal of said reradiated energy with information of saidstored message; and switching means interconnecting with said messagestorage means for altering said communications between conductorelements of said first and said second set of conductor elements. 25.The transponder according to claim 24 wherein said modulating means isresponsive to currents communicated between a pair of said conductorelements of which one element of said pair is of said first set and theother element of said pair is of said second set of conductor elements,different pairs of said conductor elements being activated in accordancewith the count of said counting means.
 26. The transponder according toclaim 25 wherein said modulating means modulates said signal ofreradiated energy with the portion of the information of said storedsignal corresponding to the pair of conductor elements activated by saidcounting means at each count of said counting means.
 27. The transponderaccording to claim 26 wherein said modulation of said signal of saidreradiated energy is a pulse code modulation.